Conventionally, a motor vehicle includes a main electronic control module (BCM or Body Control Module) for controlling a certain number of items of equipment in the vehicle. Among this equipment mention may be made, for example, of the electrical system for controlling the windows of the vehicle, the electrical control system for the rear view mirrors, the air conditioning system, the vehicle immobilization system, the central locking system etc.
For safety reasons, as shown in FIG. 1, it is known to connect the electronic control module BCM to a ground M, for example, of the vehicle battery, by way of two connecting branches: a first connecting branch B1 of logic impedance ZMI and a second connecting branch B2 of power impedance ZMp.
It is also known to link other equipments to the first connecting branch B1 and to the second connecting branch B2 in order to connect them to the ground M too.
Thus, conventionally, still with reference to FIG. 1, secondary electronic control modules ECUs (“Electronic Control Units”), such as, for example, the dashboard, an electronic door module, etc., are linked to the ground M by the first connecting branch B1 at the level of a control point PC.
Similarly, power equipments of the vehicle EQP, for example a windscreen wiper, are linked to the ground M via the second connecting branch B2 at the level of a power point Pp. Such power equipments EQP can, for example, be equipments of the vehicle engine.
To avoid damaging the electronic control module BCM, it is known to arrange on the second connecting branch B2, between the electronic control module BCM and the power point Pp of the power equipments EQP, a unidirectional current-conducting means of the diode D1 type so that the strong currents I(power) flowing from the power equipments EQP toward the ground M do not travel back up along the second connecting branch B2 to the electronic control module BCM.
In order to preserve the electronic components of the electronic control module BCM, it is known to diagnose the loss of connection of the first branch B1 between the electronic control module BCM and the ground M. Such a diagnosis is performed by a diagnostic circuit CD linked to the second connecting branch B2 at the level of the power point PP of the power equipments EQP. Indeed, when a loss of connection to the ground occurs on the first connecting branch B1, the current originating from the electronic control module BCM no longer passes through the first connecting branch B1 but passes entirely through the second connecting branch B2 through the diode D1. The loss of connection between the electronic control module BCM and the ground M on the first connecting branch B1 is therefore diagnosed by measuring the voltage difference on the second connecting branch B2.
To do this, as illustrated in FIG. 1, the diagnostic circuit CD includes a first resistor R1 assembled in series with a second resistor R2 connected to a direct current power supply VN, for example 5 V, and a microcontroller (not represented), the input of which is linked to a diagnostic point Pdiag situated between the first resistor R1 and the second resistor R2 and which defines an input voltage Vdiag. The first resistor R1 and the second resistor R2 constitute what is known as a “voltage divider”.
When the first connecting branch B1 is connected to the ground M, the diagnostic voltage Vdiag(CONN) is given by the formula:
                              Vdiag          ⁡                      (            CONN            )                          =                                            V              N                        ×                                          R                ⁢                                                                  ⁢                1                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                              -                      ZMI            ×                          (                                                I                  ⁡                                      (                    ECUs                    )                                                  +                                  I                  ⁡                                      (                    BCM                    )                                                              )                        ×                                          R                ⁢                                                                  ⁢                2                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                                                        (        1        )            
When the first connecting branch B1 is disconnected from the ground M at the level of a disconnection DIS as illustrated in FIG. 2, the diagnostic voltage Vdiag(DISCONN) is given by the formula:
                              Vdiag          ⁡                      (            DISCONN            )                          =                                            V              N                        ×                                          R                ⁢                                                                  ⁢                1                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                              -                      UD            ⁢                                                  ⁢            1            ×                                          R                ⁢                                                                  ⁢                2                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                                                        (        2        )            
where UD1 is the voltage defined across the terminals of the diode D1.
The microcontroller periodically measures the input voltage Vdiag and compares it to a threshold. When the input voltage Vdiag is above the threshold, the connection via the first branch B1 of the electronic control module BCM to the ground M is performed. When the input voltage Vdiag is below the threshold, the microcontroller diagnoses a loss of connection to the ground of the electronic control module BCM on the first branch B1.
A problem is raised if the value of the input voltage when the first branch B1 is connected Vdiag(CONN) given by equation (1) and the value of the input voltage when the first branch B1 is disconnected Vdiag(DISCONN) given by equation (2) are both very close to the threshold, i.e. when their difference is not sufficiently large for the microcontroller to be able to diagnose the loss of connection to the ground M of the electronic control module BCM on the first connecting branch B1, when the latter has indeed taken place. Such a diagnostic error leads to a lack of reliability of the device, which is a significant drawback.
This scenario occurs for current ranges of values of the voltage UD1 across the terminals of the diode D1, of the first resistor R1, of the second resistor R2 and of the currents I(BCM) and I(ECUs) flowing through the first connecting branch B1 originating from the main electronic control module BCM and/or the secondary electronic control units ECUs respectively.
Thus, for example, for conventional values of:
                    R        ⁢                                  ⁢        2                              R          ⁢                                          ⁢          1                +                  R          ⁢                                          ⁢          2                      =    0.75    ,          ⁢            UD      ⁢                          ⁢      1        =          0.1      ⁢                          ⁢      V        ,          ⁢      ZMI    =          0.3      ⁢                          ⁢      Ohms        ,          ⁢            I      ⁡              (        BCM        )              =          0.1      ⁢                          ⁢      A        ,          ⁢            I      ⁡              (        ECUs        )              =          0.2      ⁢                          ⁢      A        ,we have:
            ZMI      ×              (                              I            ⁡                          (              ECUs              )                                +                      I            ⁡                          (              BCM              )                                      )            ×                        R          ⁢                                          ⁢          2                                      R            ⁢                                                  ⁢            1                    +                      R            ⁢                                                  ⁢            2                                =          0.07      ⁢                          ⁢      V        and            UD      ⁢                          ⁢      1      ×                        R          ⁢                                          ⁢          2                                      R            ⁢                                                  ⁢            1                    +                      R            ⁢                                                  ⁢            2                                =          0.075      ⁢                          ⁢              V        .            
In this case, the values of Vdiag(CONN) and Vdiag(DISCONN) are so close to each other that the microcontroller cannot determine with certainty if the value of the input voltage Vdiag measured is above or below the threshold, which leads to frequent diagnostic errors.